Electron emitting electrode



Nov. 19, 1957 E. A. TAFT ETAL 2,813,991

ELECTRON EMITTING ELECTRODE Filed Nov. 5, 1952 Mfll/CT/NG CATHOOE f BASE IA cowl/arms FILM I Alt/00E /7 SOURCE 14070.5 73481.5 K/l. 0VOL 7'4 65 Fig.5.

Inventor-s: Ernest A.Ta1-t, Le R0 Apker;

Th ei-r- Attown ea.

United States Patent ELECTRON EMITTING ELECTRODE Ernest A. Taft and Le Roy Apker, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application November 5, 1952, Serial No. 318,784

11 Claims. (Cl. 313-94) This invention relates to an electron emitting electrode; more particularly, the invention relates to an electrode composed of a photoconducting semiconductor having a point at which electron emission concentrates.

It is an object of this invention to provide an electron emitting electrode in which emission is concentrated at a point.

It is another object of this invention to provide an electron emitting electrode which is highly responsive to small changes in light intensity or X-rays or other radiation.

It is a further object of this invention to provide a photoelectric cell having a photoconducn'ng semiconductor cathode in which electron emission is concentrated at a point.

In accordance with one embodiment, our invention is directed toward an electron emitting electrode comprising a photoconducting semiconductor of elongated configurai tion terminating in a point. Such an electrode may be incorporated in a photoelectric cell or similar light-responsive device.

incorporating the electrode of this invention. Fig. 2 is an enlarged view of the electron emitting portion of the electrode illustrated in Fig. 1, including a coil utilized to sharpen the point of the electrode. Fig. 3 is a further enlargement of the point of the electrode of Fig. 2 showing equipotential lines surrounding the point.

In the photoelectric cell of Fig. 1, an electrode 10 is mounted on a base 11 which also serves as a lead wire and is positioned preferably at a point near the center of an evacuated glass bulb 12 which may desirably be of spherical configuration. Lining the interior of a portion of the bulb 12 is a film of light-passing conducting material such as titanium dioxide or tin oxide. The film 13 may also be composed of a metal such as aluminum or silver thin enough to transmit light but thick enough to be conducting. Such a metal film may be applied as by vacuum coating of the metal from the vapor state. The base 11 is preferably a conductor connected to the exterior terminal 14 while the exterior terminal 15 is connected to the film 13 by means of a lead 16.

The leads 14 and 15 are connected through a source of energy 17 which is preferably of the order of several kilovolts potential. In the photoelectric cell thus described above, the electrode 10 is the electron emitting cathode While the film 13 of conducting material constitutes the anode. This reverses the usual photoelectric cell construction wherein the cathode emitter is coated on the wall of the bulb and the anode is positioned at the center of the bulb.

The electrode 10 is composed of a photoconducting semiconductor material of elongated configuration terminating in a point 18. Photoconducting semiconductor materials in general may be used as materials of construction for the electrode 10. Examples of satisfactory materials are the sulfide of cadmium, zinc, antimony, and molybdenum, the oxide of cadmium and zinc, the selenide of cadmium and zinc, and lead telluride. These materials provide an electron energy structure which imparts photoconductivity. It has been theorized that the degree of photoconductivity is enhanced by the presence of trace amounts of impurities which are normally present. In preparing the semiconductor materials specified above, slight deviations from stoichiometry may be sufiicient to impart photoconductivity to the final product.

The semiconductor materials may be made by a number of methods known to the art. For example, photoconductivity may be imparted to compounds by firing them for periods of more than a half hour at temperatures of the order of 700 deg. C. or higher up to the vaporization temperatures of the substances. The sulfides or selenides may be prepared by reacting the metals in vapor form with hydrogen sulfide or hydrogen selenide vapor in a hot quartz tube. In some cases, a small amount of impurity, such as silver or copper, has been added to the semiconductor material prior to firing.

We prefer that the final semiconductor have a resistivity of the order of 10 to 10 ohm centimeters in the dark. When subjected to light the resistivity is preferably lowered by a factor of or more. In thus specifying the resistivity range of the emitter material, We do not mean to indicate that there is anything critical about this range. We have found this range satisfactory when the semiconductor material is utilized in a photoelectric cell such as is illustrated in Fig. 1. For other applications different resistivity ranges may Well prove to be more useful.

An elongated rod of semiconductor material may be brought to a point by mechanical abrasion or chemical reaction; or a point may be provided by the method illustrated in Fig. 2. The rod is supported by the base 11 and it is positioned in a vacuum jar 24, the tip of the rod which it is desired to point being surrounded by a coil of wire 19. The coil 19, which constitutes a resistance heating element, is energized by voltage source 25 to provide thermal electrons which emanate from the coil and bombard the semiconductor material. Under this bombardment the semiconductor material sublimes rapidly and the end becomes pointed as shown at 18.

In operation, electron emission from the electrode 10 is concentrated at the point 18. In the photoelectric cell of Fig. l a potential of 3000 volts across the source 17 creates a field of the order of 10 volts per centimeter at the surface of the electrode 10. In the dark, the point 18 emits less than 10- amperes. However, when it is illuminated by visible light, a field emission of 20 to 100 or more times 10* amperes, depending upon the intensity of the source, is obtained. The light-sensitive region is concentrated in the tip region of electrode 10.

We have found cadmium sulfide to be a preferred material of construction for the electrode 10. In a photoelectric cell wherein the electrode 10 was composed of cadmium sulfide the application of 12 kilovolts to the circuit from adjustable voltage source 17 in the dark produced a field emission of 10- amperes. When a spot of light from a two-Watt zirconium arc was focused on the tip of the needle, a field emission of l0- amperes was drawn at 6 kilovolts potential.

In Fig. 3 a number of equipotential lines 21, 22 and 23 have been drawn around the tip 18 of the electrode 19 to represent the equipotential surfaces which have a smaller curvature than the surface of the needle tip itself because of current flow through the high resistance of the needle material. These equipotential surfaces become important when the voltage drop through the electrode material is an appreciable fraction of the applied voltage. Thus, the field at the needle tip is smaller than that for a metal similarly positioned due to the differences in conductivity. This deficiency in field, which is superposed on that due to voltage loss or drop through the electrode 10, decreases when the needle resistance is reduced by illumination. The field emission accordingly increases.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. Therefore, we aim in the appended claims to cover all such equivalent Variations as come Within the true spirit and scope of the foregoing disclosure.

What we-claim as new and desire to secure by Letters Patent of the United States is:

1. An electron emitting electrode comprising a semiconductor selected from the group consisting of the sulfide of cadmium, zinc, antimony, and molybdenum, the oxide of cadmium and zinc, the selenide of cadmium and zinc, and lead telluride having an elongated configuration terminating in a'point.

2. An electron emitting electrode as claimed in claim 1 in which the semiconductor is cadmium sulfide.

3. A photoelectric cell comprising a cathode composed of a photoconducting semiconductorot elongated configuration terminating in a point, and an anode in spaced relation with said cathode, the space between said anode and cathode being evacuated.

4. A photoelectric cell comprising a cathode composed of a semiconductor selected from the group consisting of the sulfide of cadmium, zinc, antimony, and molybdenum, the oxide of cadmium and zinc, the selenide of cadmium and zinc, and lead telluride, said cathode having an elongated configuration terminating in 'a point, and an anode in spaced relation with said cathode, the space between said anode and cathode being evacuated.

5. A photoelectric cell as claimed in claim 4 in which the semiconductor is cadmium sulfide.

6. A photoelectric cell comprising a cathode composed of a photoconducting semiconductor of elongated configuration terminating in a point, and an anode film surrounding at least a portion of said cathode and in spaced relation thereto, the space between said anode and cathode being evacuated.

7. A photoelectric cell comprising a cathode composed of a semiconductor selected from the group consisting of the sulfide of cadmium, zinc, antimony and molybdenum, the oxide of cadmium and zinc, the selenide of cadmium and zinc, and lead telluride, said cathode having an elongated configuration terminating in a point, and an anode film surrounding at least :a portion of said cathode and in spaced relation thereto, the space between said anode and cathode being evacuated.

8. .A photoelectric cell as claimed in claim 7 wherein the cathode is composed of cadmium sulfide.

9. A photoelectric cell comprising a cathode composed of a photoconducting semiconductor of elongated configuration terminating in a point, and a spherical light-passing anode film surrounding said point and spaced therefrom, the space between said anode and cathode being evacuated.

10. A photoelectric cell comprising an evacuated glass container, a cathode composed of a photoconductingsemiconductor of elongated configuration terminating in a point positioned within said container, an anode positioned within said container in spaced relation to said cathode, and exterior terminals for said anode and cathode.

ll. A photoelectric cell comprising a spherical evacuated glass container, a cathode composed of a photoconducting semiconductor of elongatedconfiguration terminating in a point positioned within said container with the point at about the center of the sphere, a light-passing anode coating on the interior 'Wall of said container, and exterior terminals for said cathode and anode.

References Cited in the file of this patent UNITED STATES PATENTS 2,401,737 I Janes June 11, 1946 2,422,041 'R'amo June 10, 1947 2,423,998 Schantz on July 15, 1947 2,444,915 "Cade July 13, 1948 2,518,048 Moore Aug. 8, 1950 2,582,850 Rose Jan. 15, 1952 2,584,461 James et a1. Feb. 5, 1952 2,640,901 Kinman June 2, 1953 2,678,400 McKay May 11, 1954 

1. AN ELECTRON EMITTING ELECTRODE COMPRISING A SEMICONDUCTOR SELECTED FROM THE GROUP CONSISTING OF THE SULFIDE OF CADMIUM, ZINC, ANTIMONY, AND MOLYBDENUM, THE OLXIDE OF CADMIUM AND ZINC, THE SELENIDE OF CADMIUM AND ZINC, AND LEAD TELLURIDE HAVING AN ELONGATED CONFIGURATION TERMINATING IN A POINT. 